Screwing around with my brew troller I've messed with PWM duty cycles up to 10hz just fine.
The issue becomes the SSR. The element is just a resistor and can switch just fine up to high frequency and still work. The SSR however when you tell it to shut off it doesnt shut off until the zero crossing of the 60hz signal coming out of the wall, so you can only ever get down to 1/120 of a second accuracy for your duty cycle control.
Thus if you're running much under 1hz and using 0 - 100% you will have no change in output power between say 1-5 % and then have a single half wave power change once you cross the proper timing, so your control resolution is reduced due to the SSR once you get much below 1hz PWM period.